The use of protective coatings for copper-containing surfaces is well known. Coatings may be applied for such purposes as to prevent tarnishing, to improve/preserve solderability, to improve corrosion resistance, and to reduce contact resistance between mating parts.
Printed circuit substrates for electronics applications, for example, generally include copper connectors such as copper pads for the mounting of surface-mount type electronic devices and/or copper plated through holes for receiving the leads of pin-in-hole type electronic devices. The surfaces of such connectors are commonly provided with a lead-tin alloy coating in practice. Tin-containing coatings are problematic, however, in that they may degrade with time due to the formation of copper-tin intermetallics. These intermetallic compounds form at the boundary between the copper and tin-containing coating and grow through the coating. The inter-metallic compounds form even at room temperature. Heating, such as during soldering, serves to accelerate the process. The problem of intermetallic compound formation becomes even more significant in connection with emerging electronics manufacturing technologies, wherein printed circuit substrates are subjected to multiple heating cycles.
Organic coatings, such as imidazoles, benzimidazoles, and triazoles have been considered as possible alternatives to lead-tin coatings. However, such coatings are subject to oxidation or volatilization when exposed to the heat of electronics manufacturing processes, with a resultant loss of protection. Moreover, organic coatings fall short of hole-fill requirements after wave soldering, particularly with the use of no-clean type soldering fluxes which are seeing increasing use in the electronics industry.